Active material actuators have long been used to do work in response to an activation signal. For example, where Martensitic shape memory alloy (SMA) wire actuators are heating to a prescribed temperature, the material is caused to undergo phase transformation, and the wire to contract in length. By contracting in length, the wire produces a driving force that can be used to manipulate a load. A common concern with SMA and other active material actuation, however, is overloading (i.e., applying an excess load above the structural capacity of the actuator) and in the case of SMA, overheating (i.e., applying an excess of heat energy above what is required to actuate the wire), which may result from overloading. These conditions may damage the actuator. It is further appreciated that elevated stress levels may damage the composing assembly or driven device whether generated by the actuator or externally.
To protect against overloading, conventional active material actuation systems typically use mechanical springs, sometimes in combination with a micro-switch or photo-interruptor, to alleviate excess stress in the active material component. These mechanisms, however, present various concerns in the art, including, for example, increased overall actuator size, and increased overload thresholds over the stroke of the actuator, which may further damage the actuator. With respect to the latter, it is appreciated that the linear increase in spring modulus as a spring-based mechanism stretches causes a proportional increase in the overload force required for further displacement. Accordingly, there is a need in the art for a more compact and/or force-reducing method of providing overload protection to active material actuators.